The present invention relates to liquid crystal displays and is directed more particularly to a method for increasing the apparent speed at which characters may be written on such displays.
Because of their thinness and low energy consumption, liquid crystal displays are frequently substituted for cathode ray tube displays in computerized data entry and display terminals. This substitution is occurring in spite of the fact that liqiud crystal displays which display both graphics and characters must use matrix addressing circuits to drive a plurality of individually addressable row and column conductors. Such addressing circuits have the disadvantage that they write characters only relatively slowly. One reason for this slow writing speed is the large number of picture elements or pixels which must be addressed in order to write a character. Another is the relatively long time time is required to change the optical properties of the liquid crystal material that is associated with each pixel. This slow writing speed is particularly apparent in electrically addressed displays that use liquid crystal materials which exhibit a storage characteristic. This is because each pixel of such displays may have to receive as many as 12 cycles of a suitable writing voltage before it becomes visible to a human observer.
When displays of the above type are used to display characters that are being typed on a keyboard by a fast operator, the rate at which the display can write characters will often be exceeded by the rate at which the operator can type characters. Assuming, for example, that the display requires 150 milliseconds to write a character, and that the operator is typing characters at an 8 character per second rate, the displayed characters will fall beyond the typed characters by 25 milliseconds for each character that is typed. Under such conditions, the display may still be writing the middle portion of a line when the operator has reached the end of that line. Because of such delays, an operator who wishes to check the correctness of an entry can be required to waste time waiting for the displayed characters to catch up with the typed characters. Even larger delays can occur when the characters being displayed are being received via a low speed modem.
Prior to the present invention, efforts to eliminate the above described delays have focused on the use of liquid crystal materials that can change state rapidly enough to allow characters to be written at the desired rate. Known liquid crystals materials that can change states rapidly enough must, however, be periodically rewritten or refreshed. This is because such liquid crystal materials do not exhibit a storage characteristic that allows them to remain continuously visible. As a result, terminals that used fast-responding liquid crystal materials had to either divert a part of the processing resources of the terminal from other processing tasks or be provided with special refresh circuitry.